In Situ Measurements of the Acoustic Target Strength of Cape Horse Mackerel Trachurus Trachurus Capensis Off Namibia

Abstract

The acoustic target strength (TS) of Cape horse mackerel Trachurus trachurus capensis was measured in situ at 38 kHz during two surveys over the Namibian continental shelf in 1998 and 1999 using a SIMRAD EK500 echosounder/ES38D submersible split-beam transducer. Scattered aggregations of horse mackerel 100–200 m deep were ensonified. The transducer was lowered to a depth of 85–140 m in order to resolve single targets at short ranges (5–50 m). Individual fish were tracked using specially developed software. Samples of ensonified fish were obtained using pelagic and demersal trawls; the former was fitted with a codend Multisampler for depth-specific sampling. Recorded TS estimates were low, producing b 20-values ranging from −77.5 to −74.9 dB (−76.0 dB ± 1.3), considerably lower than published estimates for horse mackerel (−73.4 dB < b 20 < −65.2 dB). An explanation for the weak acoustic backscattering may be swimbladder compression. Surface-projected b 20 values, which were computed using the depth of each target and the scattering area reduction rate previously found for herring (γ=−0.29), corresponded to −72.6 dB. This value is close to the TS constant of −72 dB currently applied for horse mackerel in Namibian and Angolan waters.